(1) Field of the Invention
The present invention relates to the field of simulations performed on board a rotorcraft in order to train a pilot. The present invention relates more particularly to such a simulation relating to training a pilot for the event of a failure of one of the engines of a rotorcraft having a plurality of engines, in particular a twin-engine rotorcraft.
(2) Description of Related Art
Rotorcraft are rotary wing aircraft in which at least lift is obtained by means of at least one main rotor with a substantially vertical axis. Specifically in helicopters, the main rotor provides the rotorcraft not only with lift, but also with propulsion and/or guidance in pitching and in roll. Rotorcraft are also fitted with anti-torque devices providing guidance in yaw, such as at least one auxiliary rotor with an axis that is substantially horizontal. By way of example, such an auxiliary rotor is a tail rotor or a propulsive propeller for a helicopter with high-speed propulsion.
The rotor(s) are driven in rotation by a power plant of the rotorcraft. It is common for such a power plant to comprise at least two engines, gas turbines in particular. The engines are engaged on a power transmission train interposed between the power plant and the members of the rotorcraft that consume mechanical power, in particular the rotor(s).
Typically, operation of the engine members depends on a regulation unit. Regulation of the current operation of engine members is performed by the regulation unit in compliance with a nominal mode of operation commonly referred to as all engines operative (AEO) mode.
The problem of a failure of one of the engines in a power plant needs to be taken into account in the field of aviation. In the event of such a failure, the number of engines available to deliver the required mechanical power to the rotorcraft is reduced.
That is why so-called one engine inoperative (OEI) modes have been established for regulating operation of the engines in the event of one of them failing. In the event of failure of one engine, at least one other available engine operating in OEI mode delivers a setpoint power for a predefined duration, in order to enable the rotorcraft to fly temporarily despite one of the engines being unavailable.
Various OEI modes are usually established for various flight stages of the rotorcraft, such as for example, the following current OEI modes:                OEI mode of very short duration, in which the setpoint power is delivered for a short duration of the order of 30 seconds while the rotorcraft is in a takeoff stage;        OEI mode of short duration, in which the setpoint power is delivered for a short duration of the order of two to three minutes while the rotorcraft is in the advanced stages of takeoff; and        OEI mode of long duration, in which the setpoint power is delivered for a potentially unlimited duration.        
The ability of engines to deliver the setpoint power on their own in OEI mode implies that they are overdimensioned, depending on the structure of the rotorcraft and on the operating capacities of the engines. It may be observed that it is economically useful to restrict such overdimensioning, the advantage of which is confined to the exceptional event of a failure of one of the engines.
In addition, it may be considered that piloting a rotorcraft in the event of a failure of one of the engines is difficult and requires the pilot to have specific knowledge. That is why there is a need to train pilots for such a failure.
Pilot training is frequently performed by simulation carried out in real flight on board a rotorcraft under the constraints of favorable flying conditions. To this end, a failure of one of the engines is simulated without, however, making the engines inoperative so as to keep all of the engines available and enable any one of them to be used individually in the event of a real failure of another one of them.
In the context of such a simulation, it is preferable to avoid operating an individual engine in compliance with the constraints imposed by the OEI modes. Operation of an engine in OEI mode is damaging to the engine and it is useful to preserve the engine in order to avoid costly maintenance operations. That is why in the context of a said simulation it is common practice to modify the limits imposed by the OEI modes and/or to modify the values of the criteria taken into account in order to define those limits, such as the weight of the rotorcraft.
A training mode referred to as a “single-engine” training mode is known, in which a first engine is used to deliver the setpoint power. At least one second engine is kept in operation and engaged on the power transmission train, but in desynchronized manner such that its use is reserved for maintaining drive of the main rotor to avoid it dropping below a threshold minimum speed of rotation.
If need be, and in particular if the first engine is incapable of delivering the power necessary for driving the main rotor, the second engine is available to deliver power in addition to and/or as a substitute for use of the first engine.
Such an approach is hardly satisfying as regards the speed with which the second engine can be put into use in the event of an emergency, such as in the event of a real failure of the first engine.
The desynchronization of the second engine leads to an unsatisfactory response time for resynchronizing it with the power transmission train, where such synchronization is necessary for driving the main rotor in order to enable the rotorcraft to continue flying under favorable flying conditions. Furthermore and as mentioned above, in order to avoid damage, it is not desirable in the context of a simulation to stress an engine to the limits set by an OEI mode.
Thus, another approach has been proposed in which all of the engines are used simultaneously in order to simulate a failure of one of the engines. In the context of execution of a simulated OEI mode for training purposes, the setpoint power to be delivered is shared evenly between all of the engines, which are kept synchronized with the power transmission train. Under such circumstances, each of the engines delivers mechanical power at a level corresponding to the setpoint power divided by the number of engines.
This other approach presents the advantage of keeping each of the engines in operation and in synchronous engagement with the power transmission train. In the event of a real failure of one of the engines, the reactivity of another engine to delivering all of the power needed is satisfactory. Performing simulation in this way is preferable for preserving the engines and for enabling a pilot to be trained in favorable flying conditions.
In addition, display means indicate the speed of rotation of the main rotor to the pilot so as to enable the pilot to maintain drive to the rotor so as to comply with a setpoint speed of rotation.
In the event of a reduction in the speed of rotation of the main rotor, the pilot varies the collective pitch of the blades of the main rotor so as to reduce the load on the rotor and re-establish rotor drive in compliance with the setpoint speed of rotation.
In order to understand known approaches relating to various in-flight simulations of an engine failure on-board a rotorcraft fitted with a plurality of engines, reference may be made for example to documents US 2005/234689 (PRATT & WHITNEY CANADA), and U.S. Pat. No. 5,873,546 (SIKORSKY AIRCRAFT CORP.). More specifically, relating to delivery of a setpoint power by a plurality of engines together in the context of an in-flight simulation of a failure of one of the engines, reference may be made to documents U.S. Pat. No. 4,831,567 (PRATT & WHITNEY CANADA) and U.S. Pat. No. 6,917,908 (BELL HELICOPTER TEXTRON Inc et al.).
Consideration should also be given to a potential drop in the speed of rotation of the main rotor caused by the inability of the trainee to manage a sudden simulation of a failure of one of the engines or caused under the effect of certain rotorcraft maneuvers performed by the pilot under training. As an indication, it is generally accepted that such a drop in the speed of rotation is of the order of 95% of the setpoint speed of rotation and/or by analogy is of the order of 5% per revolution of the main rotor.
In such flight, a safety device spontaneously interrupts the in-flight simulation, and each of the engines is returned to operating at its nominal power by being regulated in the AEO nominal mode of operation. Reference may also be made to documents US 2009/186320 (RUCCI J. et al.), U.S. Pat. No. 3,930,366 (NELSON R. E.), U.S. Pat. No. 5,948,023 (SIKORSKY AIRCRAFT CORP.), or also to document U.S. Pat. No. 5,873,546 (EVANS C. W. et al.).
However, it is observed that such a solution tends to generate “jolt” phenomena in yaw, thereby making it difficult for the inexperienced trainee to pilot the rotorcraft subjected to such “jolts”. It is more particularly observed that interrupting the simulation causes a sudden surge in the mechanical power delivered by the power plant, which tends to destabilize the yaw behavior of the rotorcraft.
The pilot faced with an unexpected interruption of the simulation may be surprised and unable to stabilize the rotorcraft quickly. In addition, the “jolt” phenomenon should be avoided in order to preserve the rotorcraft. It should also be considered that such a situation in which the rotorcraft is destabilized is a consequence of the unrealistic flying behavior induced by the unexpected interruption of the simulation, which goes against the educational purpose of providing a pilot with engine-failure training.
Still concerning educational purpose, it is not appropriate for the trainee to be suddenly confronted with a feeling of failure.
In addition, such a loss of stability of the rotorcraft in yaw depends on values defined by the OEI modes and on the inertia of the tail rotor. Consequently, it is usually necessary to define specific manners of operating the engines in pilot training mode corresponding to the specific structures of rotorcraft in respective families. Consequently, it appears useful to define manners of carrying out the OEI modes in the context of simulating a failure of one of the engines, which manners are suitable for use with rotorcraft of any family.
In conclusion, it appears to be useful to perfect the manners of operating the engines in order to train a pilot for a failure of an engine of a rotorcraft fitted with a plurality of engines, by taking into account as far as possible all of the constraints and requirements and/or advantages as mentioned above.